U.S. Pat. No. 4,043,285, U.S. Pat. No. 4,294,185, and U.S. Pat. No. 5,183,305 describe cargo vessels suitable for sea transport functions, which vessels have upper level cargo handling equipment and support structures for the loading and unloading of cargo containers. Such vessels, known as “Twinstar”, maximize the load transport capacity, minimize idle time in ports and provide an extremely cost-effective system for the loading and unloading of cargo. The integrated container cranes on the vessels make it possible to achieve optimal efficiency in ports. By effecting saving manpower and time, they specifically decrease or even obviate the need for manual labor for loading and unloading vessels. In addition, a large amount of the transported cargo can be stored in a safe way with a minimum of time spent and with as little labor as possible. Also, loading and unloading operations can be safely carried out even in bad and windy weather conditions.
Furthermore, U.S. Pat. No. 6,230,640 describes a refrigeration arrangement suitable for the purposes mentioned above, which, first of all, includes heat insulation in the surrounding structures of a cargo vessel's hold, and, second, refrigeration units placed in the refrigeration areas that connect to the cargo holds. Thus, it is possible to stack a large volume of containerized cargo in the holds, while keeping it apart from the refrigeration units. The cargo containers supporting the cargo are equipped with numerous ventilation openings in order to direct the refrigeration air both around the cargo and through the containers' internal and external surfaces, and then back again to the refrigeration units.
Although cargo ships of the type described above are extremely efficient in handling regular containers, it has been found that they can also be further used more extensively than before for the transportation of heavy and/or over-width cargo, i.e., for units wider than regular containers. Specially designed heavy-cargo vessels have traditionally been used, where such ships are typically loaded using cranes on the dock, which then load the cargo from the side or from the stern by floating it.
Thus, the traditional heavy-cargo vessels have been designed specifically for transporting heavy units, which means that the areas in the cargo holds are built accordingly. For this reason, the transportation of regular containers on those vessel types is not possible without special arrangements. Therefore, container transportation is usually handled using regular container vessels, which use, for the loading and unloading of cargo, container guides in the holds specially designed for that purpose.